Nowadays, as solder materials for mounting the electronic parts on the substrate, solder material of Sn—Ag system, particularly 96.5Sn-3Ag-0.5Cu (that has composition of Sn96.5 wt %, Ag3 wt % and Cu0.5 wt %) is mainly used (for example, see Japanese Patent No. 3027441 and U.S. Pat. No. 5,520,752).
FIG. 6 is a sectional view illustrating constitution of a bonding structure of the electronic circuit board using conventional solder materials.
In a case of FIG. 6, a semiconductor element 24 such as a BGA (Ball Grid Array) package or an LGA (Land Grid Array) package, and an electronic part 22 are bonded to a first mounted substrate 25 by solder 21. The semiconductor element 24 such as a BGA package or an LGA package, as shown in FIG. 6, has a sealing structure which is sealed by sealing material 23 for reinforcing, because a crack is prone to occur in a temperature cyclic ageing test or a drop test when a bonding part becomes fine.
However, in a case where the sealed first mounted substrate 25 is mounted on a second substrate 27 by using the solder of 96.5Sn-3Ag-0.5Cu, the conventional structure has a problem of causing poor connection due to melting of the solder 21 on the first mounted substrate 25. Further, reference numeral 26 denotes the bonding part which is soldered by the solder of 96.5Sn-3Ag-0.5Cu.
Therefore, a need is increasing for reducing a bonding temperature when the first mounted substrate is mounted on the second substrate 27.
Accordingly, for the purpose of preventing the solder 21 on the first mounted substrate 25 from melting by lowering the bonding temperature at which the first mounted substrate 25 is mounted on the second substrate 27, Sn—Bi system solder having a low temperature melting point, which is lower than the melting point of the solder 21 used on the first mounted substrate 25, attracts attention as a bonding material for mounting the first mounted substrate 25 on the second substrate 27 (for example, see Japanese Patent No. 4135268).